It is frequently necessary to provide power supplies to explosion-protected electrical subsystems, and particularly to such electrical subsystems which are retained within a flame-proof, or explosion-proof housing. If electrical signals are to be transmitted, that is, electrical energy at voltage and current levels which are so low that no sparking, capable of causing ignition, may occur upon connection or disconnection, or the like, or which are so low that no appreciable heating of components results, ordinary inherently non-explosion-protected connections may be used. Such connections are termed "intrinsically safe" connections. If considerable energy, however, is to be transferred, connection or disconnection of a power supply to the explosion-protected subsystems may require transfer of voltage and/or current levels which are high enough to be dangerous if the connection arrangement is located in a hazardous area. Connections then must meet the standard "flameproof enclosures" of IEC 79-1, to which ANSI C 33-30 an C 33-27 correspond.
Reference in the application will be made hereinafter to various safety standards. These standards are promulgated by the International Electrotechnical Commission (IEC) of which the U.S. is a party; local are promulgated by the Fire Underwriters Laboratory (UL), the National Electric Code (NEC), the National Electrical Manufacturers Association (NEMA) and the American National Standard Institute (ANSI). Other standards are promulgated by the European Committee for Electrotechnical Standardization (CENELEC), headquartered in Brussels, Belgium.
The standards to which particular reference will be made in this application are contained in the basic standard IEC 79, and especially in the following:
IEC 79-1 "Flameproof Enclosures", corresponding to ANSI C33-30 and C33-27
IEC 79-2) "Pressurized Enclosures"
IEC 79-13) ANSI 106.1
IEC 79-11 "Intrinsic Safety i" ANSI 4193:
Electrical apparatus for potentially explosive atmospheres.
IEC 79-7 "Increased Safety e" No corresponding ANSI:
Electrical apparatus for potentially explosive atmospheres, increased safety.
IEC 79-5 "Sand Filling q" No corresponding ANSI
IEC publication 529 (1976) "Classification of Degrees of Protection Provided by Enclosures"
IP 54: "Protection Against Splashing Waters, Dust and the Like"; roughly equivalent to NEMA 3S.
The ANSI Standards and IEC include the following definitions:
"Intrinsically Safe Circuits `i`" (ANSI 4193): a circuit in which any spark or thermal effect produced either in normal operating conditions or in specified fault conditions is incapable, under the test conditions specified in the relevant standard, of causing ignition of a specified explosive atmosphere.
"Increased Safety Electrical Apparatus `e`" (IEC79-7): electrical apparatus which does not, under normal operating conditions, produce arcs, sparks or heating likely to cause ignition of the explosive atmosphere for which it is designed and in the construction of which measures have been taken to avoid, with a major degree of security, the occurence of these phenomena under normal operating conditions and recognized overload conditions.
"Flame-Proof Enclosure `d`" (ANSI C33-27 and C33-30): an explosion-proof enclosure; a type of protection in which the parts which can ignite an explosive atmosphere and enclosed in an enclosure which can withstand the pressure developed during an internal explosion of an explosive mixture and which prevents the transmission of the explosion to the explosive atmosphere surrounding the enclosure.
"Flame-Proof Joint": a place where corresponding surfaces of the different parts of a flame-proof enclosure come together to prevent the transmission of an internal explosion to the explosive atmosphere surround the enclosure.
"Gap of a Flame-Proof Joint": the distance between the corresponding surfaces of a flame-proof joint; for a cylindrical surface, the gap is the diammetrical clearance, that is, the difference between the diameters.
"Length of Flame-Proof Joint": the length of the flame path, or the shortest path through a flame-proof joint from the inside to the outside of the flame-proof enclosure (does not apply to threaded joints).
The foregoing definitions were prepared by Technical Committee 31, "Electrical Apparatus for Explosive Atmospheres" of the IEC, and published as document 31 VEI/IEV 426 by the Secretariat of the IEC Section 123.
Explosion-protected housing for electronic subsystems or units are described in the Siemens catalog MP 29, part 10, pages 10 to 14, June 1984. This catalog describes an arrangement in which a chamber is formed by a housing. The chamber retains a circuit board with the electrical structural elements to be protected. The chamber, with the circuit board therein, is filled with stand. Electrical connections to the sand-encapsulated circuit board are formed by a connection chamber which is part of the housing, and which is constructed in accordance with the protection mode "increased-safety." It uses, for example, screw clamp connection for connection wiriting. Securely connected connection lines extend from the screw clamps into the sand filled chamber, where they are, in turn, connected to the circuit board. The pass-through to the sand filled chamber may be constructed in accordance with appropriate safety provisions, e.g., IEC Safety Provision IP 54.
This structure is inherently quite simple and has the particular advantage that the housing, because of the sand encapsulation, needs to have only slight wall thicknesses, which are much less than in housings that, with the same volume, are made in the "flame-proof encapsulation" construction. Moreover, the sand filling improves the dissipation of heat from the interior of the housing, as compared with the air filling in a flame-proof encapsulated housing. As a result, with this housing, the electronic components can be operated at a higher power loss.
However, it has been found in practice that electronic components do fail from time to time and must therefore be replaced. In the event of malfunction tracing and diagnosis as well, the particular component under observation must sometimes be disconnected from the remainder of the circuit. With the known modular housing, the subsystem or at least that part of the subsystem that contains the particular modular housing must be disconnected from voltage, before the "increased safety" connection chamber can be opened safely, in order to disconnect the supply lines in it.
In systems that contain the known modular housing, the system or corresponding part of that system must therefore be shut down for a relatively long period of time during malfunction tracing and diagnosis or malfunction clearing, because the supply of current is also shut down for the other modular housing, which actually are performing properly. Not only must those parts of the system that are connected to the same current supply as the module to be replaced be shut down, but also those parts from which or to which signal lines lead that are not considered "intrinsically safe", as defined by IEC Standard 79-11 or ANSI 4913, even though under some circumstances that system could continue to be operated at least on a limited basis even with the malfunctioning electronic component removed.